The objective of this proposed research is to make available by chemical synthesis clinically useful quantities of vitamin D3 metabolities, in particular the proposed biologically active form of the vitamin 1,25-dihydroxycholecalciferol, and several new analogs of the vitamin. The development of synthetic routes leading to the production of 1,25-dihydroxycholecalciferol (1,25-diOH-CC), other metabolities of vitamin D3 such as 25,26-dihydroxycholecalciferol (25,26-diOH-CC) and 21,25-dihydroxycholecalciferol (21-25-diOH-CC), and new analogs of the vitamin like 1-hydroxycholecalciferol (1-OH-CC) is proposed. A tabulation of the chemical and structural characterisitics of these substances and of the synthetic intermediates leading to them will be made. One synthetic plan involves the use of the Diels-Alder adduct derived from provitamin-D3 with 4-phenyl-1,2,4-triazoline-3,5-dione as a starting point. Several alternative schemes are also discussed. Each compound produced will be biologically assayed and compared with standard amounts of cholecalciferol (vitamin D3) in terms of its ability to (a) stimulate the intestinal absorption of calcium and (b) stimulate bone calcium mobilization as measured by serum calcium elevation. The availability of new analogs at the molecular level of the mechanisms of action of vitamin D and its metabolites.